I know I seldom do two posts in a day, but in addition to Rich's guest article, I wanted to point to a very interesting article Sander, one of our designers, wrote on the Excel blog.
The article is focused around the charting experience, but he posted some of the screenshots of the early prototypes we used to test Office 2007 UI designs in general.
These wire-frames, mockups, and paper prototypes were all used before we had written any code at all, as a way of evaluating how successful a design direction might be. Sander gives his perspective on how the iterative design process we used helped to shape the charting user experience. I think you'll find it interesting.
Keen eyes will notice many things that are similar to how they are in the product today, and many concepts which have changed.
Anyway, at some point in the not-too-distant future, I plan to post a set of prototypes for posterity (and for your enjoyment), but I thought Sander's article might help whet your whistle in the meantime.